Field of the Invention
The present disclosure relates to image forming apparatuses, such as a copier, a printer, a facsimile machine, and a multifunctional printer that form images employing an electrophotographic method.
Description of the Related Art
In recent years, colorization in image forming apparatuses, such as a printer and a copier, is in progress. For example, as an image forming apparatus employing an electrophotographic method, the following image forming apparatus employing an intermediate transfer system and an in-line system is well known. That is, in the above image forming apparatus, a plurality of image forming units that each include an electrophotographic photoconductor (a photosensitive member) is disposed in a line on the surface of an intermediate transfer body in a moving direction of the intermediate transfer body. Furthermore, toner images with different colors formed on the photosensitive members of the image forming units are sequentially superposed on and transferred (primarily transferred) to the intermediate transfer body. Subsequently, the multiply superimposed toner images on the intermediate transfer body are transferred (secondarily transferred) together onto a transfer material.
In the image forming apparatus employing the intermediate transfer system, residual toner (residual secondary transfer toner) is generated on the intermediate transfer body after the toner image is secondarily transferred from the intermediate transfer body to the transfer material. Accordingly, a removing process of the residual secondary transfer toner is performed so that residual secondary transfer toner does not hinder formation of the next image on the intermediate transfer body.
As an intermediate transfer body cleaning member that removes the residual secondary transfer toner, a configuration in which residual secondary transfer toner on the intermediate transfer body is scraped off by providing a cleaning blade, which is a tabular member formed of an elastic body, in an intermediate transfer body cleaning device is well known. While a low-cost and satisfactory cleaning performance can be expected from the cleaning method, the method is susceptible to unevenness of the surface of the intermediate transfer body, and there are cases in which a satisfactory cleaning performance cannot be maintained owing to the residual secondary transfer toner passing through the cleaning blade around the uneven portion.
Furthermore, one with the following configuration described in Japanese Patent Laid-Open No. 2009-139442 is well known. That is, an intermediate transfer body cleaning device is configured to include a toner charging device that charges the residual secondary transfer toner on the intermediate transfer body to a polarity that is opposite the normal charging polarity of the toner. In such a configuration, the residual secondary transfer toner that has been charged to a polarity that is opposite the normal charging polarity of the toner is reverse transferred to the photosensitive member from the intermediate transfer body at a portion immediately after the primary transfer portion of the image forming unit. The residual secondary transfer toner that has been reverse transferred is collected by a photosensitive member cleaning device that cleans the photosensitive member. In other words, it is possible to clean the intermediate transfer body by reverse transferring the residual secondary transfer toner from the intermediate transfer body to the photosensitive member, at the same time as the toner image is primarily transferred from the photosensitive member to the intermediate transfer body. Hereinafter, the above is referred to as collection at transfer. When the residual secondary transfer toner is charged to a polarity that is opposite the normal charging polarity of the toner with the charging device, the value of the generated charging current needs to be controlled to an optimum value, and the optimum value of the charging current is determined in view of faulty cleaning and negative ghost.
Description of faulty cleaning will be given first. When the absolute value of the triboelectricity (the charge amount per unit weight of the toner) of the toner is lower than a desired value when the toner is being collected by the photosensitive member, there are cases in which faulty cleaning occurs in the next image that is being formed when continuous image formation is performed.
In other words, the cleaning of the intermediate transfer body by collection at transfer is performed with the photosensitive member and through the electric field between the photosensitive member and the intermediate transfer belt by collecting the residual secondary transfer toner that has been charged to have a positive polarity that is a polarity opposite to the normal charging polarity (a negative polarity) of the toner. Accordingly, toner with a weak positive polarity (in other words, toner with a small absolute value) or toner with a triboelectricity having a negative polarity is not collected by the photosensitive member at the primary transfer portion and the toner that has not been collected by the photosensitive member unfavorably remains in the image that is in formation. Accordingly, there are cases in which the residual secondary transfer toner of the previous image creates an image defect in the image that is in formation. In order to prevent such a phenomenon, a charging current that enables the triboelectricity of the toner being collected by the photosensitive member to have a positive polarity and a suitable value needs to be distributed to the charging device.
A description of the negative ghost will be given next. When the absolute value of the triboelectricity of the toner when the toner is being collected by the photosensitive member is higher than a desired value, when continuous image formation is performed, there are cases in which negative ghost occurs in the next image that is being formed.
The toner that has been charged to have a positive polarity with the charging device and that has a high triboelectricity (in other words, toner with a large absolute value), unfavorably, electrostatically absorbs the toner (negative polarity) of the next image formed on the photosensitive member when being collected by the photosensitive member at the primary transfer portion. Furthermore, the absorbed toner of the next image unfavorably returns to the photosensitive member without being primarily transferred to the intermediate transfer body. As a result, since the toner at a portion corresponding to the previous image is unfavorably returned to the photosensitive member, a difference in density is created in the image that is being formed. With the above, there are cases in which a negative ghost that has a thin density is observed in the portion on the image that is being formed corresponding to where the residual secondary transfer toner of the previous image exists. Particularly, a negative ghost tends to occur when the charge polarity of the residual secondary transfer toner is a positive polarity and when the tirboelectricity is high (in other word when the absolute value is large). In order to prevent such a phenomenon, a charging current that enables the triboelectricity of the toner being collected by the photosensitive member to have a positive polarity and a suitable value needs to be distributed to the charging device.
As described above, with the collection at transfer method, if the charging current distributed to the charging device is large, the negative ghost worsens and when small, faulty cleaning, on the other hand, worsens; accordingly a charging current that prevents both need to be set.
As regards another cleaning member, there is a so-called hybrid cleaning method described in Japanese Patent Laid-Open No. 2014-119464 in which a cleaning blade is provided upstream and a charging device is provided downstream. In the above method, since most of the residual secondary transfer toner is mechanically scraped off with the cleaning blade on the upstream side, the amount of toner (passed-through toner) that is supplied to the charging device on the downstream side is small. Accordingly, compared with the cleaning device configured with only the charging device, the amount of toner adhering to the charging device is smaller and there is an advantage in that the time needed to clean the charging device, a so-called downtime, can be reduced.
However, it has been found that in the hybrid cleaning method described above, when the lifetime of the device is increased, a new problem described below is met.
When the lifetime of the device is increased, the time in which the charging device charges the passed-through toner increases accordingly. As a result, due to deterioration in the current carrying capacity of the charging device, an increase in resistance occurs making it difficult for the charging current needed to charge the passed-though toner of flow; accordingly, there are cases in which faulty cleaning occur.